1. Field of the Invention
The invention relates to the production of ethanol by the fermentation of carbohydrate-containing materials and, more particularly, to a method and apparatus for producing ethanol in which a drive-through pit is utilized for fermentation.
2. Description of the Prior Art
Ethanol (C.sub.2 H.sub.5 OH), also known as grain alcohol or ethyl alcohol, is a colorless, volatile liquid probably most popularly known as the physiologically active ingredient in alcoholic beverages. Ethanol has been produced for centuries by the fermentation of carbohydrates.
In the fermentation process for the production of ethanol, complex sugar, starch, hemicellulose and cellulose molecules are broken down, or hydrolyzed, into simpler components by the action of acids or enzymes. Enzymes are complex organic compounds that originate in living organisms. The presence of either acids or enzymes acts as a catalyst in the fermentation process.
The production of ethanol from substances such as corn, potatoes, rice, and the like involves first the enzymatic conversion of starch into sugar (glucose) using the enzyme amylase. The sugar then is converted into ethanol and carbon dioxide by the action of zymase, an enzyme produced by living yeast cells. Fermentation must be carried out in dilute water solutions because the yeast cells cannot live and multiply in concentrated sugar or alcohol solutions. The dilute alcohol solutions thus produced can be dehydrated to obtain concentrated solutions of water-free ethanol. Dehydration commonly is carried out by distillation, but it also can be done by the use of semipermeable membranes and chemical dewatering compounds.
The present day production of ethanol is carried out principally by acting upon only the starch-containing (or grain) portion of various crops such as corn, rice, rye, and so forth. Although the starch-containing portion of a plant generally constitutes only a small portion of the overall bulk of the plant, ethanol has been derived almost exclusive only from the starch-containing portion because starches are readily converted to glucose. The remainder of the plant, including its stalk and leaves, largely is made of hemicellulose and cellulose, both of which are relatively difficult to convert to fermentable sugars such as glucose. Hemicellulose is a branched carbohydrate polymer made up pentoses such as xylose, arabinose, and various sugar acids. Cellulose is a complex polymeric carbohydrate which has the same chemical formula as starch, but which has a slightly different molecular arrangement; the crystalline structure structure of cellulose, in combination with a polyphenolic macromolecule known as lignin contained in a plant, acts to seal the cellulose and prevent it from being hydrolyzed. Even though hydrolysis of hemicellulose and cellulose is relatively difficult, both hemicullulose and cellulose, like starch, eventually can be completely converted to fermentable sugars such as glucose and thereafter to ethanol. Reference is made to Ladisch, M.R., Cellulosic Residues (Biomass) as a Renewable Source of Fuels, American Society of Agricultural Engineers, presented at the Hyatt Regency Hotel, New Orleans, La., Dec. 11-14, 1979, the disclosure of which is incorporated herein by reference.
While the chemical reactions in the fermentation process for the production of ethanol have been known for some time, and while other processes exist for manufacturing ethanol (such as from ethylene, or as a by-product of hydrocarbon synthesis from carbon monoxide and hydrogen, among other techniques), none of the presently known techniques for manufacturing ethanol have been completely responsive to recent needs. Because of its volatile qualities, ethanol, either mixed with gasoline or used undiluted, can be used to power internal combustion engines. Due to recent shortages of petroleum, there is a great need for ethanol to be produced in quantity. In contrast to the relatively small quantities of ethanol which have been produced for beverages and other purposes, the expected widespread use of ethanol as a fuel for internal combustion engines will require a great increase in the production of ethanol in the near future.